Sizing Batteries SS1 Solar Photovoltaic Installation & Maintenance Lesson Note

Think of a battery like a water tank in your house.

• During the day, the solar panels (the pump) fill the tank.
• At night, you use the water to bath and wash dishes (your appliances).

If your tank is too small, you will run out of water before the next morning. If your battery is too small, your lights will go off at 2:00 AM. To size a battery correctly, we need to know three things:

1. Daily Load: How much “water” do we use?
2. Depth of Discharge (DoD): How much “water” can we take out without damaging the tank?
3. Autonomy: How many days should the tank last if it doesn’t rain?

Step 1 – Calculating Daily Load

Before picking a battery, we must calculate the Watt-hours (Wh) we need at night. This is our “Night Load.”

Example:

• 2 Fans (60W each) for 8 hours = 120W×8=960Wh
• 5 Bulbs (10W each) for 10 hours = 50W×10=500Wh
• Total Night Load = 1,460Wh

This 1,460Wh is the “amount of electricity” we need the battery to hold for us until the sun comes up tomorrow.

Step 2 – Depth of Discharge (DoD)

This is the most important “Safety Rule” in solar. You should never empty a battery completely. If you do, the chemicals inside will spoil, and the battery will “die” within a few months.

• Lead-Acid/Tubular Batteries: You should only use 50% of the power. If you have a 100Ah battery, you should only take 50Ah out.
• Lithium Batteries: You can use about 80% to 90% of the power safely.

The Lesson: If you need 1,000Wh of power at night, you actually need to buy a 2,000Wh Lead-Acid battery so that you only use half of it.

Step 3 – Battery Autonomy

Autonomy is a fancy word for “Independence.” It means: How many days do I want my lights to stay on if the sun doesn’t shine at all?

In Nigeria, we have many cloudy or rainy days in July and August.

• If you choose 1 Day of Autonomy, and it rains all day, your batteries won’t charge, and you will be in darkness that night.
• Most people design for 1.5 to 2 Days of Autonomy.

To do this, we simply multiply our Daily Load by the number of days.

• 1,460Wh (Daily Load) × 2 Days = 2,920Wh.

Summary and The Final Calculation

Now we bring it all together to find the Amp-Hours (Ah)—the number written on the battery.

The Formula:

Battery Size (Ah)=Battery Voltage×DoDTotal Load×Days of Autonomy​

Example:

• Load: 1,460Wh
• Autonomy: 1 Day
• Voltage: 12V
• DoD: 0.5 (50%)

Calculation=12×0.51,460×1​=61,460​=243.3Ah

The Decision: Since you can’t find a 243Ah battery easily, you would buy a 300Ah battery (or two 150Ah batteries connected together).

Class Activity: The “Rainy Day” Planner

1. The Comparison: If a Lithium battery allows 90% DoD and a Tubular battery allows 50%, which one allows you to buy a “smaller” battery for the same amount of work?
2. The Group Task: Calculate the battery size needed for a small shop that uses 500Wh at night, wants 2 days of autonomy, and uses a 12V battery with 50% DoD.
3. Discussion: Why do you think many people in Nigeria complain that their “batteries don’t last,” even when the batteries are brand new? (Hint: Think about DoD!).